Atrial arrhythmias and supra ventricular tachycardias, such as atrial fibrillation, atrial flutter and atrio-ventricular reentries, are a common postoperative complication among patients who have had heart surgery. See, for example, Cardiac Surg. Kirklin J. W., Barrat-Boyes B. C. (Eds.): NY 1993, pg. 210. During the first 10 days after heart surgery it is estimated postoperative supra ventricular tachycardia occurs in up to 63 percent of patients. See, for example, "The Importance of Age as a Predicator of Atrial Fibrillation and Flutter After Coronary Artery Bypass Grafting" Leitch et al., J. Thorac. Cardiovasc. Surg., 1990:100:338-42; "Atrial Activity During Cardioplegia and Postoperative Arrhythmias", Mullen et al., J. Thorac. Cardiovasc. Surg., 1987:94:558-65.
The presence of these arrhythmias, which in an otherwise healthy patient may not be unduly serious, may be especially harmful to heart surgery patients. The hemodynamic condition of these patients is often already compromised by either the surgery itself or the effects of prolonged anaesthesia or both. Supra ventricular tachycardias may further cause a very irregular ventricular rate which may even further deteriorate their hemodynamic condition. Such further deterioration is especially serious for patients with a compromised left ventricular function. These complications may present a serious impediment to the recovery of the patient. See, for example, "Maintenance of Exercise Stroke Volume During Ventricular Versus Atrial Synchronous Pacing: Role of Contractility" Ausubel et al., Circ., 1985:72(5):1037-43; "Basic Physiological Studies on Cardiac Pacing with Special Reference to the Optimal Mode and Rate After Cardiac Surgery" Baller et al., Thorac. Cardiovasc. Surg., 1981:29:168-73.
Due to the serious and potentially life threatening nature of these conditions, postoperative treatment is often aimed at preventing arrhythmias, such as through drugs. Drugs, however, have been found to not always be effective at preventing arrhythmias. Thus it is often necessary to provide a means for terminating any arrythymias which may occur. One common method used has been through over-pacing.
For example Waldo et al. in "Use of Temporarily Placed Epicardial Atrial Wire Electrodes For The Diagnosis and Treatment of Cardiac Arrhythmias Following Open-Heart Surgery, . "J. Thorac. Cardiovasc. Surg., 1978, vol 76, no. 4, pgs. 558-65 discloses the use of a pair of temporary heart wires placed on the atrium to diagnose and treat arrhythmias by antitachy overdrive pacing. Specifically the temporary heart wires were sutured to the atrial wall at the time of the heart surgery. Once the patient was ready to be released the wires were removed by traction or pulling upon their external end.
Temporary postoperative atrial and ventricular pacing with temporary heart wires has been found to successfully treat many of the potential post-operative arrhythmias. As such the procedure has become widespread--at least 100,000 procedures per year. Several problems, however, were encountered with the system disclosed by Waldo et al., referred to above. One problem was the stability of the heart wire within the atrial wall. Because the wall undergoes constant motion, the temporary heart wire lead was found to dislodge an unacceptable amount. Secondly, the relatively thin atrial wall, especially on elderly patients, was sometimes torn by traction upon the lead for removal.
An improved method of temporarily affixing heart wires onto the atrium was achieved with the introduction of the Medtronic Model 6500 Temporary Myocardial Pacing Lead System. That lead system featured a silicone atrial fixation disk to fasten the lead to the atrium. Specifically the silicone atrial fixation disk was permanently sutured to the atrium. The lead was positioned so that it was trapped between the disk and the atrial tissue. The lead could thereby be removed by simply pulling it from between the disk and the tissue. The rubber disk remained in the body after removal of the electrodes. The advantages offered by such a fixation system included more reliable lead fixation along with protecting the relatively thin atrial walls from tearing during lead removal. Thus the Medtronic Model 6500 Temporary Myocardial Pacing Lead permitted post-surgical temporary antitachy over-drive pacing to be performed more safely.
In spite of the improved systems or methods to achieve antitachy overdrive pacing it is not, however, always effective in terminating postoperative atrial arrhythmias or supra ventricular tachycardias. When drugs and over-pacing are not effective in the prevention or termination of postoperative supra ventricular tachycardias, or because of main negative inotropic side effects relatively contraindicated, it may become necessary to perform atrial defibrillation, synchronized to the R-wave of the electrogram, to terminate these potentially life-threatening arrythmia. Because of the large energies involved for defibrillation, however, the temporary heart wires could not be used.
External atrial defibrillation, although an effective treatment, has profound side effects. First it should be noted that in contrast to ventricular defibrillation, where conversion to normal sinus rhythm is required at the first shock, atrial defibrillation may be obtained after several shocks because ventricular contraction continues during supra ventricular tachycardia. In addition, due to the high energy required (40 to 360 Joules), the application of shocks, besides their number, is not tolerated well by a conscious patient. Therefore external defibrillation is preferably performed under general anaesthesia or at least sedation. Of course the use of anesthesia gives rise to another risk to the patient.
External defibrillation requires relatively high energy because the electrical source is not positioned directly upon the cardiac tissue but rather must pass through the thorax, which tends to dissipate the energy. In contrast, internally applied atrial defibrillation, such as may occur during surgery through defibrillation paddles placed directly on the heart, requires considerably less energy because the defibrillation electrical energy is applied only to the tissue that needs to be defibrillated. In fact, direct atrial defibrillation may be accomplished with only 1.0 Joule pulses in contrast to the 40 Joule and greater pulses for external defibrillation. See, for example, Kean D., NASPE abs. 246, PACE, April 1992, pt. II, pg. 570.
It should be understood the defibrillation success rate is dependent on the delivered energy. The lower the energy, the lower the success rate and the higher the number of shocks to be applied to obtain defibrillation success. With direct atrial defibrillation, because the energy may be applied directly to the heart, the energy level can be chosen such that both the shock level as well as the number of shocks required may be tolerated by the patient.